There are many situations in electronic applications where an AC or pulse signal of interest is superposed on a comparatively large substantially DC signal. As used herein the term “direct current” and the abbreviation DC are intended to include any current signal that is slowly varying compared to the signal of interest. Similarly, the term “alternative current” and the abbreviation AC are intended to include any current signal that is varying rapidly compared to a background DC signal. The term AC is intended to include pulse signals and vice versa.
The classical method of removing DC from a composite DC+AC signal is to use an inductor equivalent or switched capacitor circuits. The problem is exacerbated when the AC component is very much smaller in magnitude than the DC component and the composite signal source is a photodiode or other current source component. To implement these solutions in integrated circuit form is complex and/or requires large chip areas, which are undesirable.
In situations where photodiodes or the like are used to detect small optical signals in the presence of relatively large ambient light levels the problem of separating a very small AC (e.g., pulse) signal from the large ambient DC signal can be especially severe. Consider the example a situation where the ambient light causes a background DC signal from a photodiode of about 500 microamps and the AC component desired to be detected is a 100-200 picoamps pulse superimposed on the 500 microamps DC component. This AC pulse signal is about 107 times smaller than the DC ambient signal.
In many cases in order to do further processing on the pulse signal it also often needs to be converted to a voltage. The traditional methods for doing this involve passing the photodiode current though a resistor, either directly or via a feedback resistor associated with an op-amp. FIG. 1 shows prior art op-amp circuit 1 used to convert current Is in current source 2 to voltage Vout at output terminal 8 of circuit 1. Feedback resistor Rfb is connected between nodes 6 and 7. Node 6 is connected to current source 2 and the negative input of op-amp 3. Node 7 is connected to output terminal 8 of op-amp 3. Voltage Vref is connected to terminal 4 and the positive input of op-amp 3. Op-amp 3 is coupled to power supply line 5 having supply voltage Vdd thereon.
When current source 2 provides composite current Is with a large DC component Idc (e.g., 0.5×10−3 amps) and a small AC component Ip (e.g., 2×10−10 amps), i.e., Is=Idc+Ip where Idc>>Ip, then it is very difficult to convert Ip to a significant Vout. The voltage Vfb across feedback resistor Rfb cannot exceed power voltage Vdd. Thus, Rfb(max) is given by Vdd/Is. Consider the situation where Vdd is approximately 3 volts, Is˜Idc=0.5 milliamps for Ip=200 picoamps. Then Rfb(max)=Vdd/Is=(3/0.5×10−3)=6×103 ohms. This results in a very small AC voltage Vp(out) from current-to-voltage converter 1, that is. Vp(out)=Ip×Rfb=2×10−10×6×103=1.2×10−6 volts. If the supply voltage for the IC is lower, the value of Rfb must be correspondingly reduced and the Vp(out) corresponding to Ip will be further reduced. When the AC signal to be post processed after current-voltage conversion is superimposed on a large DC signal, the AC voltage output with the prior art arrangement is only a very small fraction of the power supply voltage, making subsequent signal processing more difficult.
Vp(out) of the current voltage converter illustrated in FIG. 1 scales linearly with Rfb, while the noise voltage V(noise)=(4kT*Rfb*Δf)½ scales only with the square root of the resistance. For Example, the Signal to Noise Ratio (SNR) for a given value of Ip is given by:SNR=V(signal)/V(noise)=Vp(out)/(4kT*Rfb*Δf)½=Ip*Rfb/(4kT*Rfb*Δf)½=Constant*(Rfb)½  Eq. 1where * indicates multiplication. Thus, for optimal signal/noise performance, large values of Rfb are better. But with the prior art arrangements, large values of Rfb are not possible with small supply voltages.
Accordingly, there is an ongoing need to detect small AC signals, particularly small pulse signals in the presence of a large DC ambient signal and to separate the AC signals for further processing. In addition, there is an ongoing need to provide for current-to-voltage conversion of the AC component of superimposed current signals using large resistance values. Further, there is an ongoing need to convert currents with superposed large DC and very small AC components in a manner so that large AC voltage swings can be obtained relative to the power supply voltage. In addition, there is an ongoing need to provide DC-AC current separation and conversion in a manner that is suitable for fabrication in a monolithic integrated circuit. Still further, there is an ongoing need for means and method for separating the DC and AC components of the signal current before current-to-voltage conversion. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.